The prolactins of avian, reptillian and amphibian species will be isolated and identified by pigeon crop and mammotropic assays. These prolactins will be tested for somatotropic activity by the hypophysectomized rat tibial assay and by injection into hypophysectomized individuals of the homologous species. Rat and guinea pig prolactins will be similarly isolated and assayed to provide comparisons with mammalian material which has been subjected to comparable manipulations. The results of these studies should provide information on whether the possession of lactogenic and somatotropic activities in the same hormonal entity is peculiar to primates and to sheep prolactin, or whether this phenomenon is a more general one among the vertebrates. Such data will allow the situation found with primate growth hormone-prolactin to be viewed meaningfully against a background of the evolution of adenohypophysial lactogenic and somatotropic principles.